1. Field of the Invention
This invention relates to semiconductor materials, methods, and devices, and more particularly, to the growth of nonpolar or semipolar nitride Light Emitting Diodes (LEDs) and diode lasers.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [Ref(s). x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
The usefulness of gallium nitride (GaN) and alloys of (Ga,Al,In,B)N has been well established for fabrication of visible and ultraviolet optoelectronic devices and high power electronic devices. Current state-of-the-art nitride thin films, heterostructures, and devices are grown along the [0001] axis. The total polarization of such films consists of spontaneous and piezoelectric polarization contributions, both of which originate from the single polar [0002] axis 100 of the würtzite nitride crystal structure 102, as illustrated in FIG. 1(a). When nitride heterostructures are grown pseudomorphically, polarization discontinuities are formed at surfaces (e.g. c-plane surface 104, as shown in FIG. 1(a) and interfaces within the crystal 102. These discontinuities lead to the accumulation or depletion of carriers at surfaces and interfaces, which in turn produce electric fields. Since the alignment of these polarization-induced electric fields coincides with the typical [0001] growth direction of nitride thin films and heterostructures, these fields have the effect of “tilting” the energy bands of nitride devices.
In c-plane würtzite nitride quantum wells, the “tilted” energy bands spatially separate the electron wavefunction 106 and hole wavefunction 108, as illustrated in FIG. 1(b). This spatial charge separation reduces the oscillator strength of radiative transitions and red-shifts the emission wavelength. These effects are manifestations of the quantum confined Stark effect (QCSE) and have been thoroughly analyzed for nitride quantum wells [Refs. 5-8]. Additionally, the large polarization-induced electric fields can be partially screened by dopants and injected carriers [Refs. 9, 10], making the emission characteristics difficult to engineer accurately.
Furthermore, it has been theoretically predicted that pseudomorphic biaxial strain has little effect on reducing the effective hole mass in c-plane würtzite nitride quantum wells [Ref 11]. This is in stark contrast to typical III-V zinc-blende InP-based and GaAs-based quantum wells, where anisotropic strain-induced splitting of the heavy hole and light hole bands leads to a significant reduction in the effective hole mass. A reduction in the effective hole mass leads to a substantial increase in the quasi-Fermi level separation for any given carrier density in typical III-V zinc-blende InP- and GaAs-based quantum wells. As a direct consequence of this increase in quasi-Fermi level separation, much smaller carrier densities are needed to generate optical gain [Ref. 12]. However, in the case of the würtzite nitride crystal structure, the hexagonal symmetry and small spin-orbit coupling of the nitrogen atoms in biaxially strained c-plane nitride quantum wells produces negligible splitting of the heavy hole and light hole bands [Ref 11]. Thus, the effective hole mass remains much larger than the effective electron mass in biaxially strained c-plane nitride quantum wells, and very high current densities are needed to generate optical gain in c-plane nitride diode lasers.
One approach to decreasing polarization effects in nitride devices is to grow the devices on nonpolar planes of the crystal. These include the {11-20} planes, known collectively as a-planes, and the {10-10} planes, known collectively as m-planes. Such planes contain equal numbers of gallium and nitrogen atoms per plane and are charge-neutral. Subsequent nonpolar layers are equivalent to one another so the bulk crystal will not be polarized along the growth direction.
Another approach to reducing polarization effects and effective hole masses in nitride devices is to grow the devices on semipolar planes of the crystal. The term “semipolar plane” can be used to refer to any plane that cannot be classified as c-plane, a-plane, or m-plane. In crystallographic terms, a semipolar plane would be any plane that has at least two nonzero h, i, or k Miller indices and a nonzero 1 Miller index. Subsequent semipolar layers are equivalent to one another so the bulk crystal will have reduced polarization along the growth direction.
Unlike strained c-plane InxGa1-xN quantum wells, it has been predicted that strained nonpolar or semipolar InxGa1-xN quantum wells should exhibit anisotropic splitting of the heavy hole and light hole bands, which should lead to a reduction in the effective hole mass for such structures [Ref 13]. Self-consistent calculations of many-body optical gain for compressively strained InxGa1-xN quantum wells suggest that the peak gain is very sensitive to effective hole mass and net quantum well polarization, and that peak gain should increase dramatically as the angle between a general growth orientation and the c-axis increases, reaching a maximum for growth orientations perpendicular to the c-axis (i.e. on nonpolar planes) [Refs. 14, 15].
Finally, commercial c-plane nitride LEDs do not exhibit any degree of optical polarization in their electroluminescence. Nonpolar or semipolar nitride LEDs, on the other hand, have demonstrated strong optical polarization in their electroluminescence [Refs. 16-18]. This optical polarization can be attributed to anisotropic strain-induced splitting of the heavy hole and light hole bands in compressively strained nonpolar or semipolar InxGa1-xN quantum wells, leading to significant discrepancies in the magnitude of different optical matrix elements.
However, current state of the art nonpolar or semipolar (Ga,Al,In,B)N substrates exhibit unusual surface morphologies [Refs. 2-4].
FIGS. 2(a)-(c) are taken from [Ref 2], wherein FIG. 2(a) and FIG. 2(b) are Nomarski optical micrographs showing the top surface of an m-plane n-type GaN film and m-plane LED structure, respectively, showing pyramidal hillocks 200, and FIG. 2(c) is a schematic illustration showing the cross-section of the pyramidal hillocks 200 having a height h, width w, and length l, and the m-plane GaN film is grown on nominally on-axis m-plane GaN substrates.
FIGS. 3(a)-(g) are also taken from [Ref 2], wherein FIG. 3(a) shows an amplitude image of an n-type GaN film over a 10×10 μm2 area showing four pyramid faces (two a-inclined faces a1 and a2, and two c-inclined faces c+ and c−), wherein the slope angles of the pyramids are 0.1° for the a-inclined pyramid faces and 0.5°-0.6° for the c-inclined faces, FIG. 3(b) is an amplitude image of an m-plane LED structure, FIGS. 3(c)-(f) are height images for the c−, a1, a2, and c+ inclined faces of the LED structure in FIG. 3(b), respectively, and FIG. 3(g) is a schematic showing the − and c+-inclined hillock faces were decorated with microscopic pyramids (plan view and cross-section of the hillocks in FIGS. 3(d), (e), and (f)).
FIGS. 4(a)-(b) are also taken from [Ref 2] and show a series of Nomarski optical micrographs of m-plane GaN films grown on off-axis substrates with different miscut directions and miscut angles, wherein in FIG. 4(a) the a-miscut angles range from 0° to 0.35° in 0.1° increments and 0.52° (from left to right), and in FIG. 4(b) the c− miscut angles are 0.01°, 0.45°, 5.4°, and 9.7°, from left to right, respectively.
FIG. 5 is taken from [Ref 3] and shows a surface morphology of an m-plane GaN substrate after epitaxial growth of a laser structure observed by optical differential contrast microscopy.
FIGS. 6(a)-(b) are taken from [Ref. 4], wherein FIG. 6(a) and FIG. 6(b) are Nomarski optical micrographs of m-plane GaN grown on off-angled substrate with orientations 0.2° and 2.2° toward the <0001> direction, respectively.
There is a need to provide smoother nonpolar and semipolar films. The present invention satisfies this need.